Pains in terminal cancer patients are serious problems, and it is an important object to release cancer patients from their pains to improve quality of life of the patients. Conventionally, narcotic analgesics, morphine as a typical example, have been used for pain treatments of such cancer patients. It is known that, among the types of pains in cancer, there is intractable neuropathic pain (the pain is called as “neuropathic pain” or “neurogenic pain”, and will be referred to as “neuropathic pain” in the specification) for which morphine is hardly effective, besides somatic pain caused by noxious stimulus in peripheries such as mechanical stimulus, chemical stimulus and thermal stimulus, and visceral pain caused by stimulus such as dilatation of membranes due to traction or enlargement of parenchymal organs and elevation of internal pressure of hollow organs.
Normally, pains are generated by damages of tissues at the corresponding tissue portions, and the pains will be dispersed when the tissue damages are cured. However, although no tissue damage is observed at the site of the pain, pains may sometimes be generated such as burning, constricting, pricking or electrification-like pain. Such pains are called neuropathic pains, and the pains are caused by damages or dysfunctions of peripheral or central nerves.
Although the neuropathic pain may be independently generated, it is considered that, in cancer pain, the pain is combined and mixed with somatic pains in about 30% of cases. In drug therapy of neuropathic cancer pain, antidepressants, anticonvulsants, local anesthesia, α2 agonists, GABA receptor agonists, NMDA receptor antagonists and so forth have been used as supplemental analgesic agents for narcotic analgesics and the like. However, they, per se, have severe side effects, and moreover, they may sometimes deteriorate the side effects of morphine which is administered to most of cancer patients, since they have low compatibility with morphine. Therefore, a supplemental analgesic agent has been desired which has high safety and good congeniality with morphine (Kongetsu no Chiryo (Treatment of This Month), Vol.8, No. 3, 2000, Separate Issue).
Furthermore, it is known that neuropathic pains are observed not only in cancer pains but in postherpetic neuralgia, post-thoracotomic pain, diabetic neuropathy, CRPS (complex regional pain syndrome, those in which nervous damages are not apparently observed are referred to as “type-1” (reflux sympathetic dystrophy (RSD) and those in which nervous damages are observed are referred to “type-2” (causalgia)), multiple sclerosis, AIDS, trigeminal neuralgia, thalamic pain, paraplegic pain caused by myelopathy, anesthesia dolorosa, or phantom limb pain (Igaku no Ayumi (Advance of Medicine), Vol. 195, No.9, 2000.12.2, pp.627-632).